Sensors can be found everywhere in today's society—heat sensors are utilized in a common thermostat to activate/deactivate heating and cooling units. Light sensors can be used to govern when to turn on and off automobile headlights (e.g., if the sensor does not receive a predetermined minimum amount of light, a control unit will activate headlights). Even an alarm clock can be thought of as a time sensor, wherein upon sensing a preset time a signal is output from the clock. Dynamic sensing of torque and strain in a rotating shaft would be beneficial towards improving design and analysis of machinery, as well as allowing for torque and strain of a rotating shaft to be utilized as a control parameter in a control system(s). Dynamic torque information also provides an important capability to perform real-time diagnostics for rotating machinery and diagnose mechanical faults thereby predicting and mitigating catastrophic failure. Rotating machinery is prevalent in virtually every manufacturing facility. It is not uncommon for a facility to have hundreds or even thousands of rotating machines. Commercial systems including HVAC systems, vehicles, aircraft, and ships also depend on a large number of rotating machines working reliably. Information related to a rotating system is important for control, such as in dynamometer test stands and many manufacturing processes. Furthermore, such information is important to avoid unexpected or hazardous operating conditions and catastrophic failure. Similarly, it is desirable to determine lateral and angular strain (e.g., torque) for non-rotating shafts. For example, a vehicle steering system can benefit from determining steering wheel torque applied by a driver, strain on steering linkage components, and dynamic wheel torque experienced by wheels on the road. Robust, affordable, lightweight sensing systems for obtaining data regarding a moving or rotating shaft (specifically torque), however, are presently non-existent.
Rotating shafts driving a load are susceptible to torsional strain. Continuous strain on the shaft will eventually result in machine inefficiency and/or shaft deformation and/or breakage (e.g., shaft cracking). Dynamic torque (strain) fluctuations can cause mechanical and fatigue damage as well as accelerate machine failure. A sensor used to measure torsional strain (torque) on a shaft can therefore be desirable. However, conventional torque sensors are large, costly, heavy, failure prone, and provide limited dynamic signal (bandwidth). For example, a conventional torque sensing system can weigh approximately 1500 pounds if one desired to measure torque found in a tail rotor of a helicopter. According to another prior art technique, a shaft can be instrumented with strain gauges and thus, establish power and signal coupling to the rotating shaft. However, providing suitable signal amplification and signal analysis is also impractical due to size, weight, reliability, cost, and performance factors. Furthermore, the conventional shaft torque sensing system in the example above would cost at least five thousand dollars, be susceptible to failure due to shock loads, and take up valuable aircraft space and payload capacity. Conventional sensing systems that consider angular displacement difference between two ends of a rotating shaft typically involve costly and complex optics and mechanical interconnect equipment.
Additionally, rotating shafts are prone to misalignment when two or more shafts are employed and coupled together. For example, misalignment between a motor shaft and a pump shaft is a significant and prevalent cause of premature bearing and seal failure, which often cause motor failures and motor-pump system failures. Misalignment can also cause accelerated wear on couplings, mountings, piping, and increased heating and energy loss. Conventional systems typically align coupled shafts utilizing mechanical gauges and shims; however, these techniques are not precise. Alternative techniques employ laser alignment, but these systems are typically costly, time consuming, and are only performed while the machinery is not operating. Effects such as thermal expansion and dynamic shaft loading due to mechanical or magnetic fields can cause a system aligned while at rest to become misaligned when functioning at operating temperatures with loads.
Prior art methods can also have a limited ability to verify the integrity of composite joints. Composite materials are being utilized in an increasing number of structures (e.g., aircrafts). For example, shafts constructed from composite material can be employed to carry power from a main engine to a tail rotor in a helicopter. Prior art techniques (e.g., acoustic, IR) are difficult, costly and time consuming.
Using a property of photo-elasticity in conjunction with optical sensors to measure torque on a rotating shaft is one prior art method to mitigate disadvantages of conventional torque measuring systems. Optical sensing systems are desirable because they are compact and lightweight in comparison to conventional electric or magnetic sensors, and have significantly greater immunity to electromagnetic interference as compared to many conventional systems. Furthermore, optical sensing systems can be produced inexpensively and allow for quick replacement/repair of the system. Lastly, optical sensing systems can provide high frequency torque signals unobtainable with conventional torque sensor(s).
The property of photo-elasticity has been used to measure strain on materials for over fifty years. The method of determining strain relies on the birefringence property exhibited by transparent glasses and plastics (e.g., polarized light waves exhibit a phase and angle shift when traversing through photo-elastic material under deformation). In particular, the phenomenon of load-induced birefringence is utilized where a material exhibits birefringence under deformation caused by external loading. In practice, polarized light is delivered into a photo-elastic material wherein a strain in the photo-elastic material is encountered. The normally incident polarized light is shifted in phase and angle along the principal strain directions as it propagates through a photo-elastic material. The velocities of light transmission along these directions are directly proportional to magnitude of respective principal strains. The light is then passed through a second polarizer or an analyzer, which resolves the rotated beams into a light-intensity pattern the strain on the material can be deduced by inspection of the resulting light-intensity pattern.
FIG. 1 illustrates an exemplary cross-sectional view of a prior art optical sensing system 100 which can be employed to measure torque on a rotating shaft. The sensing system 100 includes a light emitting component 102, a capturing component 104, and strips of photo-elastic material 106. The strips of photo-elastic material 106 encircle a rotating shaft 108, and interiors of the strips of photo-elastic material 106 are coated with a reflective substance such as an aluminum filled epoxy. Two strips of photo-elastic material 106 are depicted in FIG. 1 for purposes of demonstration; however, any number of strips can be employed. In operation, the light emitting component 102 releases a beam of light into a strip of photo-elastic material 106. The light reflects off of a reflective interior surface of the strip of the photo-elastic material 106, and back out through the photo-elastic material radially and into the capturing component 104. A light-intensity pattern is visible from the light exiting the strips of photo-elastic material 106. The light-intensity pattern is a spectrum of colors known in the art as a fringe pattern, wherein each fringe is comprised of a band of colors. The number of fringes, narrowness and proximity of the fringes in the fringe pattern indicate the amount of phase retardation of the transmitted beam of light and therefore, strain experienced in the photo-elastic material. This provides a measure of the amount of strain on the rotating shaft 108. Data acquisition equipment, signal processing, and pattern recognition software (not shown) are employed to determine a quantitative, accurate measure of the angular strain or torque on the shaft. The photo-elastic strip 106 is intended to encircle the rotating shaft and be axisymmetric. By imaging the photo-elastic strip at the same location or and different locations on the strip, a dynamic measure of the torque on the shaft may be obtained as the shaft rotates.
The subject invention as described below provides for significant improvements over the aforementioned prior art systems and addresses unmet needs with respect to the amount of shaft data desired in connection with rotating shafts. In particular, the prior art systems and methods fail to establish whether rotating shafts are in alignment and the degree of misalignment, and further lack the ability to perform dynamical alignment. Additionally, there is a need for improved techniques to monitor shaft cracking and verify composite joint integrity.